<?php

function custom_search( $query ) {

    if( ! is_admin() && $query->is_main_query() ) {

        if ( $query->is_search() ) {

            $meta_query = $query->get( 'meta_query' );

            $meta_query[] = array(
                'key'       => 'product_code',
                'value'     => $query->query['s'],
                'compare'   => 'LIKE'
            );

            $query->set( 'meta_query', $meta_query );

        }

    }

}

add_action( 'woocommerce_product_query' , 'custom_search' );